The present invention relates to an electrode for implantation in the hear, particularly for stimulating the heart muscle, the electrode being of a type which includes an electrode lead, a screw-like protruding helix at the end of the electrode lead near the heart for screwing the electrode into cardiac tissue, and a protective device which protects against inadvertant hooking of the helix and which protrudes over the helix during insertion of the electrode.
Such electrodes serve to transmit stimulation pulses and physiological signals between the heart muscle and a likewise implanted artificial cardiac pacemaker.
It is already known, as disclosed in German Offenlegungsschrift [Laid-Open Application] No. 26 13 044, to protect the helix by means of an elastic cuff which encloses it laterally. If, during the implantation, the electrode end directed toward the heart has been placed at a suitable location within the heart, the exertion of a force in the direction of insertion through the electrode lead and simultaneous rotation of the same, causes the helix to be screwed into the cardiac tissue while the front edge of the cuff rests on the tissue surrounding the point of puncture and is increasingly compressed during the screwing in of the structure so that the part of the helix to be screwed in is progressively exposed. However, for this purpose the electrode lead must be relatively stiff so that it can transmit not only the pressure required to screw in the helix but also the pressure required to compress the cuff. In order to prevent the occurrence of breaks, however, such electrode lead should be as soft as possible.
It is a further drawback that the cuff, once the electrode has been fixed in the heart, continously exerts a force in a direction to cause the screwed-in portion of the electrode head to be pulled out. Moreover, the formation of fibrinous tissue is enhanced so that, as a whole, there is a tendency for the electrode to become ineffective prematurely.
The cuff must also have a certain minimum stiffness since use of a comparatively soft cuff, while it would exert less force when the electrode is in its fastened state, could cause release of the helix during the insertion permitting the helix to possibly become hooked in a vein, or could result, during screwing in of the helix, in entanglement of the cuff with the helix or with the electrode tip, or in hooking of the cuff thereinto, so that this would create a blockage making further screwing in impossible.
A further drawback of the above-metioned electrode is that up to the point of fixation it is caused by the cuff so that it is impossible, before attachment, to establish electrical contact with the cardiac tissue in order to measure the excitation threshold or record an intracardial electrocardiogram.